Q: I get an error "Soap Error: fetching http headers"
A: This happens when the plugins are trying to reach your interface under http but you have it run it under https. So just change it in the config file.

In the configfile of the "ispconfig3_account" (/plugins/ispconfig3_account/config/config.inc.php) fill in your details:
$rcmail_config['remote_soap_user'] = '{REMOTE USERNAME}';
$rcmail_config['remote_soap_pass'] = '{REMOTE PASSWORD}';
$rcmail_config['soap_url'] = 'http://{YOUR SERVER}:8080/remote/';
Change the port (set to "8080" by default) if necessary and please note that when using SSL to access ISPconfig panel, use "https://" instead of "http://".

What does it say? if your ispconfig panel is accessible per https, then you have to use https in soap url....